Oleg Nesterov ce4f06dcbb stop_machine: Touch_nmi_watchdog() after MULTI_STOP_PREPARE ... Suppose that stop_machine(fn) hangs because fn() hangs. In this case NMI hard-lockup can be triggered on another CPU which does nothing wrong and the trace from nmi_panic() won't help to investigate the problem. And this change "fixes" the problem we (seem to) hit in practice. - stop_two_cpus(0, 1) races with show_state_filter() running on CPU_0. - CPU_1 already spins in MULTI_STOP_PREPARE state, it detects the soft lockup and tries to report the problem. - show_state_filter() enables preemption, CPU_0 calls multi_cpu_stop() which goes to MULTI_STOP_DISABLE_IRQ state and disables interrupts. - CPU_1 spends more than 10 seconds trying to flush the log buffer to the slow serial console. - NMI interrupt on CPU_0 (which now waits for CPU_1) calls nmi_panic(). Reported-by: Wang Shu <shuwang@redhat.com> Signed-off-by: Oleg Nesterov <oleg@redhat.com> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Dave Anderson <anderson@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Tejun Heo <tj@kernel.org> Link: http://lkml.kernel.org/r/20160726185736.GB4088@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>		2016-07-27 11:12:11 +02:00
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stop_machine.c	stop_machine: Touch_nmi_watchdog() after MULTI_STOP_PREPARE	2016-07-27 11:12:11 +02:00
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